Geophysical and geochemical site investigation of eastern part of Lagos metropolis, southwestern Nigeria

Abstract

Geophysical and geochemical studies were carried out in the eastern part of Lagos metropolis to determine the possible contamination of subsurface soils and groundwater as a result of dumping of waste into the canal. The geophysical method involved the use of 2D electrical resistivity imaging which was acquired along five (5) traverses, while the geochemical method involved the physicochemical analysis of water samples from nine (9) wells within the study area. The geochemical study includes the analysis of physical parameters like total dissolved solids (TDS), electrical conductivity (EC), pH and temperature. Seventy elements which include trace elements (such as As, B, Ba, Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, U and Zn), major cations (such as Ca, K, Mg and Na) and anions (Cl and Br) were analysed, and the concentrations of the elements were compared with the World Health Organization (WHO) and Nigerian Standard for Drinking Water Quality (NSDWQ) permissible level for drinking water. Geophysical results show that the lithology is composed of topsoil, clayey sand, peat and sand. 2D electrical resistivity tomography revealed that the leachate and polluted water from the canal have percolated to the subsurface soil down to a depth of about 37 m with low resistivity values of 1–13 Ω m. In addition to this, result of physicochemical analysis of water samples shows that Mn, Br, TDS, and EC were high in some wells, above the WHO and NSDWQ standard. It was deduced that the subsurface has been invaded by the polluted water from the canal consequently, making the groundwater unsuitable for human consumption.